Air bags are inflatable bags that remain folded and out of sight in readiness for a frontal collision. During a collision, chemical reaction of a material typically containing sodium azide produces gaseous products which inflate the bag and interpose it between the driver and the steering wheel or the front seat occupant and the dashboard.
Belt tensioners are devices that tighten automobile seat belts during a crash to hold the seat occupants more securely. A tensioner has a small motor driven by pressurized gas from the chemical reaction of gas generating material like that used for inflating air bags. In both cases, chemical reaction of the gas generating material is initiated by an electrically hated squib. The sensor of this invention may be adapted to control the power that energizes the squib.
A sensor of the type to which this invention is applicable is in commercial production by Breed Automotive Corporation. It consists of a metallic ball free to move in a sealed cylinder containing contacts that are bridged by the ball when it moves to one end of the cylinder. Air flow around the ball causes a pressure differential. The pressure differential causes a force proportional to and opposite the relative velocity of the ball with respect to the cylinder. A permanent magnet provides a bias force that retains the ball in a home position away from the contacts and causes the velocity change required for switch closure to increase with the duration of the crash pulse. The proportionality of force to velocity makes the sensor an acceleration integrator that completes the firing circuit upon achievement of a predetermined velocity change. The variation of air viscosity with temperature is compensated by making the cylinder and ball of particular and different stainless steels selected to have a difference in their thermal expansion coefficients such that the gap between the ball and cylinder changes with temperature as required to maintain the performance of the sensor over a wide temperature range.
This sensor is expensive to manufacture and one reason for the cost is the high precision required of the ball and the cylinder. Another reason for the high cost for certain applications is that the cylinder must be made of material that is difficult to machine. Further, in known designs, the contacts are connected with leadin wires and a diagnostic resistor by means such as welding or soldering. These processes create contamination during manufacturing that cannot be tolerated in the vicinity of the ball and cylinder thereby requiring additional components and processing steps to maintain isolation for cleanliness. Further, small particles in the cylinder can prevent proper operation. Therefore great care is required to insure cleanliness during manufacture and this adds to the manufacturing cost. Further, in certain crashes there is substantial cross axis vibration which causes the ball motion to not be simple linear movement down the cylinder which affects the calibration of the sensor and extraordinary measures such as vibration damping isolators and other means are required to minimize these effects and these measures are not always entirely successful.
A second sensor of the type to which this invention is applicable has been widely promoted by Breed Automotive Corporation. It is like the first except that the bias is supplied by a lever driven by a spring and, instead of bridging contacts, the movement of the ball releases a firing pin to impinge on a stab primer and thereby initiate deployment of an air bag.
Copending application Ser. No. 190165 titled Compact Crash Sensing Switch And Diagnostic System filed May 5, 1988 describes a crash sensor of the type including a ball moving in a cylinder with a semiconductor device for completing the firing circuit and also having diagnostic capabilities. The semiconductor switch and diagnostic features of that invention are applicable to the present invention and the application Ser. No. 190165 is incorporated herein by reference.
A general object of this invention is to provide a crash sensor for automotive vehicles which overcomes certain disadvantages of the prior art.